The present invention relates to a method of determining collagen fragments in body fluids. The invention further relates to means, including synthetic peptides, monoclonal and polyclonal antibodies and cell lines, for use in carrying out the method of the invention. Still further, the invention relates to the use of the above method to diagnose the presence of disorders associated with the metabolism of collagen, especially osteoporosis.
Osteoporosis is the most common bone disease in humans. Primary osteoporosis, accompanied by increased susceptibility to fractures, results from a progressive reduction in skeletal bone mass. It is estimated to affect 15-20 million individuals in the USA alone. Its basis is an age-dependent imbalance in bone remodeling, i.e., in the rates of formation and resorption of bone tissue.
In the USA about 1.2 million osteoporosis-related fractures occur in the elderly each year including about 538,000 compression fractures of the spine, about 227,000 hip fractures and a substantial number of early fractured peripheral bones. Between 12 and 20% of the hip fractures are fatal because they cause severe trauma and bleeding, and half of the surviving patients require nursing home care. Total costs from osteoporosis-related injuries now amount to at least $10 billion annually in the USA (Riggs, New England Journal of Medicine 327:620-627 (1992)).
Osteoporosis is most common in postmenopausal women who, on average, lose 15% of their bone mass in the 10 years after menopause This disease also occurs in men as they get older and in young amenorrheic women athletes. Despite the major, and growing, social and economic consequences of osteoporosis, the availability of reliable assays for measuring bone resorption rates in patients or in healthy subjects is very limited. Other disorders entailing (and correlated with) abnormalities in collagen metabolism include Paget""s disease, Marfan""s syndrome, osteogenesis imperfecta neoplastic growth in collagenous tissue, dwarfism, rheumatoid arthritis, osteoarthritis and vasculitis syndrome.
Three known classes of human collagen have been described to date. The Class I collagens, subdivided into types I, II, III, V, and XI, are known to form fibrils. Their full amino-acid sequence (to the extent they have been elucidated) are attached in Appendix A.
Collagen type I accounts for more than 90% of the organic matrix of bone. Therefore, in principle, it is possible to estimate the rate of bone resorption by monitoring the degradation of collagen type I. Likewise, a number of other disease states involving connective tissue can be monitored by determining the degradation of collagen. Examples are collagen type II degradation associated with rheumatoid arthritis and osteoarthritis and collagen type III degradation in vasculitis syndrome.
Amino acid sequences of human type III collagen, human pro 1(II) collagen, and the entire prepro 1(III) chain of human type III collagen and corresponding cDNA clones have been investigated and determined by several groups of researchers; see Loil et al., Nucleic Acids Research 12:9383-9394 (1984); Sangiorgi et al., Nucleic Acids Research 13:2207-2225 (1985); Baldwin et al., Biochem J. 262:521-528 (1989); and Ala-Kokko et al., Biochem. J. 260:509-516 (1989).
Type I, II, and III collagens are all formed in the organism as procollagen molecules, comprising N-terminal and C-terminal propeptide sequences, which are attached to the core collagen molecules. After removal of the propeptides, which occur naturally in vivo during collagen synthesis, the remaining core of the collagen molecules consists largely of a triple-helical domain having terminal telopeptide sequences which are non-triple-helical. These telopeptide sequences have an important function as sites of intermolecular cross-linking of collagen fibrils extracellularly. The alpha-helical region also includes crosslinkable sites. Peptides from this region are part of the present invention.
Intermolecular cross-links provide collagen fibrils with biomechanical stability. The formation of these cross-links is initiated by modification of lysine and hydroxylysine residues to the corresponding aldehydes. Several of these residues located on adjacent chains of collagen will spontaneously form different intermolecular cross-links. The exact position of the sites for cross-linking on collagen telopeptides and from the helical region has been previously described. See, for example, Kxc3xchn, K., in Immunochemistry of the Extracellular Matrix 1:1-29, CRC Press, Inc., Boca Raton, Fla. (1982), Eyre, D. R., Ann. Rev. Biochem. 53:717-48 (1984) or U.S. Pat. No. 5,140,103). Furthermore, the amino acid sequences of some potential sites for cross-linking in type I, II, and III collagen are given in Table 1 below.
The fibrous proteins, collagen and elastin, are cross-linked by a unique mechanism based on aldehyde formation from lysine or hydroxylysine side chains. Four homologous loci of cross-linking are evident in molecules of type I, II and III collagens (for review see Kxc3xchn, K., in Immunochemistry of the Extracellular Matrix 1:1-29 (1982)). Two are aldehyde sites, one in each telopeptide region. The other two sites are hydroxylysine symmetrically placed at about 90 residues from each end of the molecule. When collagen molecules pack into fibrils, these latter sites in the helical region align and react with telopeptide aldehydes in adjacent molecules. There is now strong evidence that 3-hydroxypyridinium residues are the mature cross-link coming from hydroxylysine-derived aldehydes. The mature cross-linking residues of the other pathway, i.e. from aldehyde formation of lysine residues, is, however, still unknown.
In the past, assays have been developed for monitoring degradation of collagen in vivo by measuring various biochemical markers, some of which have been degradation products of collagen. However, none of these methods are based upon the use of immunological binding partners in the form of antibodies which are immunoreactive with synthetic peptides having a sequence essentially derived from collagen fragments having crosslinkable sites.
For example, hydroxyproline, an amino acid largely restricted to collagen, and the principal structural protein in bone and all other connective tissues, is excreted in urine. Its excretion rate is known to be increased in certain conditions, notably Paget""s disease, a metabolic bone disorder in which bone turnover is greatly increased, as discussed further below.
For this reason, urinary hydroxyproline has been used extensively as an amino acid marker for collagen degradation; Singer, F. R. et al., Metabolic Bone Disease, Vol. II (eds. Avioli, L. V., and Kane, S. M.), 489-575 (1978), Academic Press, New York.
U.S. Pat. No. 3,600,132 discloses a process for the determination of hydroxyproline in body fluids such as serum, urine, lumbar fluid and other intercellular fluids in order to monitor deviations in collagen metabolism. The patent states that hydroxyproline correlates with increased collagen anabolism or catabolism associated with pathological conditions such as Paget""s disease, Marfan""s syndrome, osteogenesis imperfecta, neoplastic growth in collagen tissues and in various forms of dwarfism.
Bone resorption associated with Paget""s disease has also been monitored by measuring small peptides containing hydroxyproline, which are excreted in the urine following degradation of bone collagen; Russell et al., Metab. Bone Dis. and Rel. Res. 4 and 5:255-262 (1981), and Singer, F. R., et al., supra.
In the case of Paget""s disease, the increased urinary hydroxyproline probably comes largely from bone degradation; hydroxyproline, however, generally cannot be used as a specific index for bone degradation. Much of the hydroxyproline in urine may come from new collagen synthesis (considerable amounts of the newly made protein are degraded and excreted without ever becoming incorporated into tissue fabric), and from turnover of certain blood proteins as well as other proteins that contain hydroxyproline.
Furthermore, about 80% of the free hydroxyproline derived from protein degradation is metabolized in the liver and never appears in the urine. Kiviriko, K. I., Int. Rev. Connect. Tissue Res. 5:93 (1970), and Weiss, P. H. and Klein, L., J. Clin. Invest. 48:1 (1969). Hydroxyproline is a good marker for osteoporosis, but it is troublesome to handle. It is specific for collagen in bones.
Hydroxylysine and its glycoside derivatives, both peculiar to collagenous proteins, have been considered to be more accurate than hydroxyproline as markers of collagen degradation. However, for the same reasons described above for hydroxyproline, hydroxylysine and its glycosides are probably equally non-specific markers of bone resorption; Krane, S. M. and Simon, L. S., Develop. Biochem. 22:185 (1981).
Other researchers have measured the cross-linking compound 3-hydroxypyridinium in urine as an index of collagen degradation in joint diseases. See, for background and as examples, Wu and Eyre, Biochemistry 23:1850 (1984); Black et al, Annals of the Rheumatic Diseases 48:641-644 (1989); Robins et al.; Annals of the Rheumatic Diseases 45:969-973 (1986); and Seibel et al., The Journal of Dermatology 16:964 (1989). In contrast to the present invention, these prior researchers have hydrolyzed peptides from body fluids and then looked for the presence of individual 3-hydroxypyridinium residues.
Assays for determination of the degradation of type I, II, and III collagen are disclosed in U.S. Pat. Nos. 4,973,666 and 5,140,103. However, both these patents are restricted to collagen fragments containing the cross-linker 3-hydroxypyridinium, whereas the present invention does not rely on the presence or absence of this particular cross-linking structure. Furthermore, the above-mentioned assays require tedious and complicated purifications from urine of collagen fragments containing 3-hydroxypyridinium to be used for the production of antibodies and for antigens in the assays.
At present very few clinical data using the approach described in U.S. Pat. Nos. 4,973,666 and 5,140,103 are available. Particularly, no data concerning the correlation between the urinary concentration (as determined by methods described in the above-mentioned patents) of 3-hydroxypyridinium containing telopeptides of type I collagen and the actual bone loss (as determined by repeated measurements by bone densitometry) are published. The presence of 3-hydroxypyridinium containing telopeptides in urine requires the proper formation in bone tissue of this specific cross-linking structure at various times before the bone resorbing process. Very little information on these processes is available and the present invention seeks to circumvent this dependance of the correct formation of the cross-linking structure. Furthermore, preliminary data indicate that in one embodiment of the present invention a major fraction of the molecules reactive in the assay has a molecular weight of more than 4,000 daltons. On the contrary, only molecules with a molecular weight below 2.000 daltons are identified in urine with the monoclonal antibody used in the assay; Hanson et al., Journal of Bone and Mineral Research 7:1251-1258 (1992). This demonstrates that the method according to the present invention has a very different profile of reactivities, i.e. it detects very different molecules, compared to methods described in the two above-mentioned U.S. patents.
None of the above researchers have reported specifically assaying a crosslinkable collagen fragment that is naturally produced in vivo upon collagen degradation, as in the present invention.
GB patent application No. 2,205,643 reports that the degradation of type III collagen in the body can be quantitatively determined by measuring the concentration of an N-terminal telopeptide from type III collagen in a body fluid. This method does not relate to methods employing antibodies reactive with specific, low molecular weight sequences around crosslinkable structures. Instead, the method uses antibodies generated to N-terminal telopeptides released by bacterial collagenase degradation of type III collagen, said telopeptides being labelled and used in the assay.
There are a number of reports indicating that collagen degradation can be measured by quantitating certain procollagen peptides. Propeptides are distinguished from telopeptides and alpha-helical region of the collagen core by their location in the procollagen molecule and the timing of their cleavage in vivo; see U.S. Pat. Nos. 4,504,587; 4,312,853; Pierard et al., Analytical Biochemistry 141:127-136 (1984); Niemela, Clin. Chem. 31/8:1301-1304 (1985); and Rohde et al., European Journal of Clinical Investigation 9:451-459 (1979).
U.S. Pat. No. 4,778,768 relates to a method of determining changes occurring in articular cartilage involving quantifying proteoglycan monomers or antigenic fragments thereof in a synovial fluid sample. This US patent does not relate to detecting collagen fragments derived from degraded collagen.
Dodge, J. Clin, Invest. 83:647-661 (1981) discloses methods for analyzing type II collagen degradation utilizing a polyclonal antiserum that specifically reacts with unwound alpha-chains and cyanogen bromide-derived peptides of human and bovine type II collagens. Contrary to the present invention the degradation products of collagen were not detected in a body fluid, but histochemically by staining of cell cultures, i.e. by xe2x80x9cin situxe2x80x9d detection. The main difference between Dodge and the present invention is that Dodge measures type II collagen degradation in situ. By xe2x80x9cin situxe2x80x9d is meant a determination carried out in the cells or tissue in which the degradation takes place. There is quite a fundamental difference between this determination and a method based upon tracing a marker in vitro, e.g. in the urine.
None of these references specify the structures of particular telopeptide or alpha-helical degradation products that could be measured to determine the amount of degraded fibrillar collagen in vivo.
The present invention is based on the discovery of the presence of particular collagen fragments in body fluids of patients and normal human subjects. The collagen fragments are generated upon collagen degradation and are partly characterized by the presence of potential sites for cross-linking, e.g. by the presence of lysine or hydroxylysine (Kxc3xchn, K., Immunochemistry of the Extracellular Matrix 1:1-29 (1982)). It is believed that a fraction of the collagen fragments measured by the assays embodied in the present invention are normally covalently linked in vivo to other peptide fragments through different, unidentified or already identified cross-links.
The method of the present invention may be used for determination of the degradation of human collagen of type I, II, and III.
The present invention provides a method of assessing the degradation of collagen based on a determination of the presence and quantity of a particular class of collagen fragments produced in vivo upon collagen degradation; and a comparison of the detected collagen fragments to those of a predetermined standard developed by measuring the same class of collagen fragments in normal individuals, i.e. individuals not afflicted with a disorder affecting collagen metabolism, said individuals being sex- and age-matched with the subjects being tested.
The present invention uses antibodies immunoreactive with synthetic peptides without these cross-linking structures. Accordingly, it is believed that collagen fragments (corresponding to the synthetic peptides) with or without actual cross-links, but with crosslinkable sites, are measured in the assays embodied in the present invention.
In a preferred embodiment, the method is based on the competitive binding of collagen fragments in body fluids and of synthetic peptides essentially derived from collagen to immunological binding partners.
The present invention provides new and very simple procedures for the detection (qualitative and quantitative) of collagen fragments generated upon collagen degradation.
For purposes of the present invention, as disclosed and claimed herein, the following terms are as defined below:
xe2x80x9cAntibodyxe2x80x9d: A monoclonal or polyclonal antibody or immunoreactive fragment thereof (i.e. capable of binding the same antigenic determinant), includingxe2x80x94but not limited toxe2x80x94Fab, Fabxe2x80x2, and F(abxe2x80x2)2 fragments.
xe2x80x9cCrosslinkable sitesxe2x80x9d: loci in collagen telopeptide or helix amino acid sequences containing lysine or hydroxylysine residues which can form cross-links with telopeptides or helical amino acid sequences of other collagen molecules in vivo.
xe2x80x9cCrosslinkable peptidesxe2x80x9d: peptides containing a fragment of the collagen sequence including at least one crosslinkable site.
Test kit: A combination of reagents and instructions for use in conducting an assay.
xe2x80x9cEssentially derivedxe2x80x9d (about structures): Structures with similar antigenicity, i.e. with an ability, above the level of a non-related peptide, to inhibit the binding of any of the mentioned synthetic peptides to an antibody immunoreactive with said synthetic peptide.
It is contemplated that the method may also be used for assaying collagen fragments in animal body fluids, e.g. for determination of the collagen metabolism. It also can be used during clinical testing of new drugs to assess the impact of these drugs on collagen metabolism.
More specifically, the present invention relates to methods for assaying collagen fragments by the use of synthetic peptides corresponding to the above-mentioned sequences of collagen. Generally, these synthetic peptides will have fewer amino acid residues than the entire collagen molecule, often they will have fewer than 10 amino acids. Also, the synthetic peptides, corresponding to molecules present in body fluids, e.g. urine, will have potential sites for cross-linking, preferably lysine or hydroxylysine, incorporated in the structure.
The present invention comprises the determination of collagen fragments by the use of antibodies which are immunoreactive with the above-mentioned synthetic peptides, said peptides each having a sequence derived from collagen fragments having crosslinkable sites.
The invention also includes cell lines (e.g. hybridomas) that produce monoclonal antibodies immunoreactive with the above-mentioned synthetic peptides. The invention further includes monoclonal antibodies produced by the fused cell hybrids, and those antibodies (as well as binding fragments thereof, e.g. Fab) coupled to a detectable marker. Examples of detectable markers include, but are not limited to, enzymes, chromophores, fluorophores, coenzymes, enzyme inhibitors, chemiluminescent materials, paramagnetic metals, spin labels and radioisotopes.
The methods of the invention involve quantitating in a body fluid the concentration of particular collagen fragments derived from collagen degradation. In a representative assay, collagen fragments in the patient""s body fluid and a synthetic peptide immobilized on a solid surface are contacted with an immunological binding partner which is immunoreactive with the synthetic peptide. Suitable body fluids are e.g. human urine, blood, serum, plasma and synovial fluid. It is contemplated that the method may also be used e.g. on saliva and sweat. The body fluid may be used as it is, or it may be purified prior to the contacting step. This purification step may be accomplished using a number of standard procedures, including, but not limited to, cartridge adsorption and elution, molecular sieve chromatography, dialysis, ion exchange, alumina chromatography, hydroxyapatite chromatography, and combinations thereof.
The present invention is based on the discovery of simple procedures for quantitation of collagen fragments in body fluids. In a representative procedure, synthetic peptides containing potential sites for cross-linking, are used for the raising of antibodies and subsequently incorporated in the assay for quantitation of collagen fragments generated in vivo by collagen degradation.
The invention also includes test kits useful for quantitating in a body fluid the amount of collagen fragment derived from the degradation of collagen. The kits comprise at least one immunological binding partner, e.g. a monoclonal or polyclonal antibody specific for a peptide derived from the degradation of collagen. If desired, the immunological binding partner of the test kit may be coupled to detectable markers such as the ones described above.
The invention is described in more detail below. Reference is made to the appended drawings.